In the conventional dehumidification air-conditioning and method utilizing solar heat, the mechanism is large-scaled in size and low in working efficiency. Also it is difficult for the conventional method to make equal use of solar energy stored in the summer and provide in the winter. Accordingly, an object of this invention is to provide an improvement of the conventional method by using a multistage sun collector, solutions for use in providing dehumidification such as a calcium chloride solution and a vacuum vaporation process.